California Redwoods Hold Hidden History Of Coastal Climate

Giant redwood trees can provide a glimpse into historic coastal climate conditions using a novel method for analyzing the wood’s oxygen and carbon content to detect fog and rainfall in previous seasons, according to a study published in the Journal of Geophysical Research: Biogeosciences.

Study leader Jim Johnstone at the University of Washington developed the new method, and used the cores from Northern California coastal redwoods to trace the climate back 50 years.

“This is really the first time that climate reconstruction has ever been done with redwoods,” Johnstone said. Weather records corroborate Johnstone’s findings, proving that the method is accurate and suggesting that it could be used to track conditions throughout the thousand or more years of the redwoods’ lifetime.

Coastal redwoods are not the longest-lived trees on the West Coast, but they do contain unique information about their foggy surroundings.

“Redwoods are restricted to a very narrow strip along the coastline,” Johnstone explained. “They’re tied to the coastline, and they’re sensitive to marine conditions, so they actually may tell you more about what’s happening over the ocean than they do about what’s happening over land.”

Many people have used tree rings as a window into the past, but until now the redwoods were seen as too erratic in their growth patterns to be used to reconstruct historic climate. Tree-ring research, or dendrochronology, typically involves a detailed look at a cross-section of a tree trunk. However, the rings of redwoods are uneven, and sometimes fail to fully encircle the tree, making it a poor candidate for anything except detecting historic fires.

By contrast, Johnstone’s painstaking approach is more akin to processing ice cores, because it uses the molecules captured in the wood to sample the atmosphere of the past. Most oxygen in Earth’s atmosphere has an atomic mass of 16, making it O-16, but a small percentage of oxygen is the heavier O-18 isotope. When seawater evaporates off the ocean to form clouds, some drops fall as rain over the ocean, and more of the heavier O-18 molecules rain out. As a result, the remaining drops that fall on land have a higher percentage of the lighter O-16 molecules. But fog, on the other hand, forms near the shore, and blows on land where it drips down through the branches until the trees use it like rainwater.

By analyzing the proportion of O-16 and O-18 in the wood from each season, Johnstone and his team were able to measure the contribution of fog and rain. The researchers examined the spring growth from April to June, as well as the fall growth from August to October, and also analyzed carbon atoms to measure the total amount of moisture in the air.

“We actually have two indicators that we can use in combination to determine if a particular summer was foggy with a little rain, foggy with a lot of rain, and various combinations of the two,” Johnstone said.

Related research by Johnstone shows that the amount of West Coast fog is closely tied to the surface temperature of the ocean, meaning redwoods may reveal information about the long-term patterns of ocean change, such as the Pacific Decadal Oscillation. Understanding of the natural variability cycles could also help to better distinguish natural and human-caused climate change.

“It’s possible that the redwoods could give us direct indication of how that’s worked over longer periods,” Johnstone said. “This is just a piece that contributes to that understanding in a pretty unique place.”